The role of complement in the development of cancer is multifaceted and complex, and may likely vary between distinct forms. On one hand, the complement system can inhibit tumor growth through complement-dependent cytotoxicity (CDC), a mechanism that is also actively engaged by certain therapeutic antibodies. On the other hand, tumor cells express high amount of membrane-bound and fluid-phase regulators, which inhibit the activation of complement and contribute to tumor growth. Finally, certain complement effectors (e.g., the anaphylatoxin C5a) have been shown to modulate the tumor microenvironment and shape the cellular immune response. This functional map highlight some of the key interactions and mechanisms of complement in cancer pathology.